1. Field of the Invention
The present invention relates to a preparation method of DL-alpha-tocopherol through the condensation between isophytol or phytol derivatives and trimethylhydroquinone (TMHQ) in the presence of a Znxe2x80x94Al heterogeneous catalyst. More particularly, the present invention relates to a Zn(II)-coated silica-alumina synthetic catalyst which is anchored with a Zn and an Al site, simultaneously and thus are very effective in the preparation of DL-xcex1-tocopherol with high yield.
2. Description of the Prior Art
For the past few decades, extensive effort has been made to effectively prepare DL-xcex1-tocopherol by use of Zn(II) ions as metallic catalysts (Lewis acid catalyst). Conventionally, the DL-xcex1-tocopherol is prepared through the condensation of an isophytol and trimethylhydroquinone (TMHQ) represented by the following reaction formula 1: 
For example, U.S. Pat. No. 4,217,285 (hereinafter ""285 patent) discloses the synthesis of DL-xcex1-tocopherol in toluene or n-hexane solvent with ZnCl2 and silica-alumina (or silica-gel) in the presence of acid, especially HCl, asserting that tocopherol can be obtained with a purity of 95 to 96% at a production yield of 99% or higher. Also, U.S. Pat. Nos. 4,634,781 and 4,639,533, both assigned to BASF, disclose processes producing for DL-xcex1-tocopherol in which isophytol is reacted with amines such as a tridecylamine and thereafter with TMHQ in the presence of ZnCl2 and HCl, which are somewhat complicated and inefficient. In those cases, the tocopherol is described to be produced with a purity of 94 to 95% at a yield of 95 to 98%.
However, the conventional techniques leave room for improving purity because their DL-xcex1-tocopherol is as low as 95% pure on average. Particularly, the BASF patents are inefficient in that they do not satisfy the desired yield of DL-xcex1-tocopherol.
Being used as a solvent in the ""285 patent, toluene or hexane brings about a poor result in the total yield of DL-xcex1-tocopherol. When used, toluene itself is partially reacted with isophytol to be produced undesired by-products. Hexane, although not reacting with isophytol, lengthens the reaction time owing to its low boiling point (approximately 69xc2x0 C.) such that the catalyst aggravates the dehydration of isophytol. Furthermore, when the synthesis of DL-xcex1-tocopherol is carried out in such a reaction procedure, by-products analogous in structure to DL-xcex1-tocopherol are found in relatively large quantities, decreasing the purity of the DL-xcex1-tocopherol. This is made worse when acid or base is used as a co-catalyst. In the case of ZnCl2 alone, not together with an Al-based catalyst such as AlCl3 or SiO2xe2x80x94Al2O3, analogous by-products appear in abundance. Moreover, the conventional techniques suffer from the serious problem of having to treat the waste water resulting from the use of acid or base such as hydrochloric acid.
Knowledge of catalytic reaction mechanisms allows modification and adaptation leading to the present invention.
The intensive and extensive research on the preparation of DL-xcex1-tocopherol, conducted by the present inventors, resulted in the finding that a novel catalyst system obtained by coating ZnCl2 on silica-alumina through sintering is useful for us to prepare highly pure DL-xcex1-tocopherol at a high yield with ease.
The effectiveness of ZnCl2 and silica-alumina was supported by various experiments with ZnCl2 and AlCl3-mixed catalyst systems, which led to the result that the Zn(II) ion plays an important role in the synthesis of DL-xcex1-tocopherol as a catalyst while, acting as an ancillary catalyst, the Al(III) ion reduces side-reactions and increases the purity of the final product. On the whole, a Znxe2x80x94Al mixed catalyst system was found to enable a pure DL-xcex1-tocopherol to be synthesized at a high yield.
Based on this finding, the present inventors developed a Zn(II) ion catalyst system into a Znxe2x80x94Al based catalyst system which affords an ideal synthetic site for DL-xcex1-tocopherol. After extensive trials, a silica-alumina catalyst coated with Zn(II) ions was found to be the most effective in preparing DL-xcex1-tocopherol from a minimum amount of isophytol or phytol derivatives while generating substantially no by-products.
Therefore, it is an object of the present invention to provide a method for preparing DL-xcex1-tocopherol at a high yield with greater ease.
It is another object of the present invention to provide a method for preparing DL-xcex1-tocopherol, which can be applied for continuous reactions for the production of DL-xcex1-tocopherol on a commercial scale with a great convenience.
It is a further object of the present invention to provide a method for preparing DL-xcex1-tocopherol, which generates substantially no by-products, so as to use expensive isophytol or phytol derivatives at a minimum amount.
Based on the present invention, the above objects could be accomplished by providing a method for preparing DL-xcex1-tocopherol at a high yield, comprising condensation reacting isophytol or phytol derivatives with trimethylhydroquinone at 80 to 120xc2x0 C. for 2 to 7 hours in the presence of a Zn(II) ion-coated silica-alumina catalyst system in hydrocarbon solvent, particularly in n-heptane solvent.
In accordance with the present invention, DL-xcex1-tocopherol is prepared through the condensation between isophytol or phytol derivatives and TMHQ in the presence of a Zn(II) ion-coated silica-alumina heterogeneous catalyst in a non-polar solvent.
Available in the present invention is isophytol or phytol derivatives represented by the following chemical formula I or II: 
wherein X and Y are independently a hydroxy group, a halogen atom or an acetoxy group.
Fundamentally, mixed catalyst systems in the prior art suffer from the problem caused by employing component catalysts separately. In addition, they are not convenient to handle. With these disadvantages, conventional mixed catalyst systems are considerably difficult to apply for continuous processes for the preparation of DL-xcex1-tocopherol. However, the problems with the conventional catalyst systems can be overcome by the catalyst system according to the present invention, which has Zn(II) coated on silica-alumina. The suitable catalyst in the present invention is made in the form of a Zn(II) supported silica-alumina, such that is anchored with a Zn(II) and an Al(III) site, simultaneously. The coating of Zn(II) ions on the silica-alumina support is achieved by impregnating ZnCl2 into a silica-alumina support and sintering the ZnCl2 impregnated support at about 400xc2x0 C. for 2 hours. For instance, a catalyst system obtained after ZnCl2 was used at an amount of 50% by weight of silica-alumina upon the sintering, was analyzed for the composition thereof and the result is given in Table 1, below, demonstrating that ZnCl2 is found to be successfully impregnated into the silica-alumina support.
When DL-xcex1-tocopherol was prepared in the presence of this catalyst, the resulting product was found to be as good as or better than that when in the presence of ZnCl2 and silica-alumina, separately. This finding proves beyond doubt the ability of the catalyst according to the present invention to provide an ideal environment for the preparation of DL-xcex1-tocopherol.
A preferable result is obtained when the catalyst of the present invention is used at an amount of 20 to 450 weight parts based on 100 weight parts of TMHQ. The range of 100 to 200 weight parts of the catalyst brings about a more preferable result in the preparation of DL-xcex1-tocopherol. In preparing the catalyst according to the present invention, ZnCl2 is preferably used at an amount of 5 to 250 weight parts based on 100 weight parts of silica-alumina and most preferably at an amount of 20 to 80 weight parts. With respect to the catalytically effective quantity, an excellent product of tocopherol is obtained when the amount of the catalyst is maintained in a specific range to the amount of TMHQ as well as when the amount ratio of ZnCl2 to SiO2xe2x80x94Al2O3 is maintained within such a range.
Examples of the solvent useful in the present invention include aromatic solvents such as toluene, benzene and xylene and aliphatic saturated hydrocarbon solvents such as n-heptane, n-hexane and n-octane with a preference for toluene and n-heptane. Of them, n-heptane guarantees the most preferable synthesis results in the present invention.
The most preferable period of reaction time falls within the range of 2 to 7 hours, while suitable reaction temperatures are in the range of 80 to 120xc2x0 C.
In the following Table 2, there are given reaction conditions and their results.
Analysis of all the reaction products was conducted by use of gas phase chromatography (HP-5890 series) with HP-1 columns.
Summarized in Table 2 are the results obtained by the reaction equivalents of TMHQ and isophytol with each other in the two solvents in accordance with catalysts. As apparent in Table 2, the product obtained using the Zn(II)-coated Sixe2x80x94Al catalyst is almost equal to that obtained through use of a catalyst mixture of ZnCl2 and Sixe2x80x94Al in the same solvent. When toluene was used, approximately 6% of TMHQ remained unreacted as seen from the production yield of 93 to 94%. Thus, the corresponding amount of isophytol did not take part in the reaction and was found to be decomposed by the catalyst. On the other hand, n-heptane improved the production yield to at least 99%, leaving TMHQ unreacted at a level of 1% or less. Thus, 99% or more of the isophytol took part in the reaction while the corresponding amount, that is, 1% or less of isophytol was decomposed by the catalyst. This result indicates that isophytol is more stable in heptane than in other solvents. Consequently, the employment of heptane as a reaction solvent can minimize the amount of isophytol needed to completely react a given amount of TMHQ, thereby reducing raw material cost for the production of DL-xcex1-tocopherol.
A better understanding of the present invention may be obtained in light of the following examples which are set forth to illustrate, but are not to be interpreted to limit the present invention.